The present invention relates to methods and apparatus for providing a termination function that exhibits a controlled impedance to terminate a signal, such as may be used in an integrated circuit or the like.
Proper termination of high-frequency signals are of particular concern in the design and implementation of electronic circuits, such as digital computers, microprocessors, digital signal processors, memory circuits, or virtually any other electronic circuit in which impedance matching is important. Indeed, if the impedance of a receiving or transmitting circuit is not properly controlled, then undesired transmission line effects may result, such as undesirable signal reflections. Signal reflections are of particular concern in high-frequency applications, such as in digital circuits, where signal reflections may result in unwanted interference with an incident or received signal that result in an erroneous logic level.
A prior art technique of providing a termination of a desirable impedance is illustrated in FIG. 1. In particular, FIG. 1 illustrates a termination circuit 10 employing a resistor divider comprising a first resistor RA and a second resistor RB coupled in series across voltage sources Vdd and Vss. With this configuration, the impedance at the termination node is approximately equal to the parallel combination of RA and RB. The values of RA and RB are selected in order to achieve a desirable impedance, thereby reducing or eliminating unwanted signal reflections. Among the disadvantages of this prior art resistor-termination approach is relatively high cross current and attendant power dissipation.
The power consumption and dissipation characteristics of the resistor-termination approach will be discussed with reference to FIG. 2. FIG. 2 illustrates the current I and voltage V characteristics of the respective resistors RA and RB. The voltage V along the abscissa of the illustrated graph represents a voltage induced on the termination node of the termination circuit 10 of FIG. 1. The induced voltage may be produced by a driver circuit that operates to produce a logic high level or a logic low level. For example, the driver circuit might output a logic high level by placing a voltage approaching Vdd on the termination node. Alternatively, the driver circuit might operate to produce a logic low level by placing a voltage approaching Vss on the termination node. The voltage placed on the termination node by the driver circuit affects the current characteristics of the resistors RA and RB. Assuming that RA and RB are substantially equal to one another, then the currents IRA and IRB through RA and RB, respectively, intersect one another at a voltage of about (Vdd−Vss)/2.
As the voltage V at the termination node is reduced by the driver circuit, the current IRA through resistor RA increases (due to an increase in the voltage thereacross) and the current IRB through resistor RB reduces (due to a reduction in the voltage thereacross). The summation of the currents IRA and IRB is substantially equal to the shunt current Is drawn by the termination circuit 10. Depending on the values of Vdd, Vss, RA and RB, the magnitude of the shunt current Is may be on the order of about 10 mA. This is a relatively high value.
Accordingly, there are needs in the art for new methods and apparatus for terminating a signal that do not draw excessive current from a power supply, yet provide a controlled impedance in order to reduce or eliminate signal reflections.